The present invention relates to a method for the sealing of a packing material provided with an emptying opening with two thermoplastic layers located by means of heat and pressure on opposite sides of the opening to each other. The invention also relates to an arrangement by which the method is carried out.
Packing containers of the non-returnable type are frequently manufactured from laminated material which is folded and sealed so that a packing container of the desired shape is obtained. When the packing container is intended for liquid contents, e.g. milk or juice, it is then provided with some form of opening arrangement. The opening arrangement may consist of an emptying opening punched out in the top of the packing container which is provided with a covering, tear-off strip, a so-called pull-tab. The packing laminate usually comprises a central layer of fibrous material e.g. paper, which is covered on either side by thermoplastic layers. To prevent the paper layer, exposed in the cut edges around the emptying opening, from absorbing some contents it was necessary in this type of opening arrangements, in addition to the outer cover strip, to also arrange an inner, liquid-tight protective strip which is sealed to the inside of the packing material around the emptying opening. When the packing container is to be opened the emptying opening is exposed by tearing off the outer cover strip. In doing that a part of the inner protective strip corresponding to the shape of the emptying opening also has to be removed, and this is achieved simplest by sealing the cover strip and the protective strip to each other within a region corresponding to the area of the emptying opening. This design makes it possible to expose the emptying opening on tearing off the cover strip in one action, but it is found in practice that it may sometimes be difficult to break through and completely remove the desired parts of the inner protective strip. Moreover, parts of the protective strip often remain in the emptying opening, so that an edge consisting of a number of plastic shreds is formed which renders difficult the emptying out of the contents through the opening in a satisfactory manner.
The disadvantages are due, among other things, to an unsatisfactory adhesion between cover strip and protective strip which is particularly marked on the parts located along the edge area of the emptying opening. It has been endeavoured hitherto to overcome these disadvantages by using, for the sealing together of the strips and the packing material, a resilient sealing or pressure pad which thanks to its flexibility compensates for the variations in thickness which are caused by the absence of packing laminate in the actual emptying opening. This method has proved to give a satisfactory adhesion in the central parts of the emptying opening, but cannot eliminate the unsatisfactory adhesion along the edges of the emptying opening.
It is a general object to provide a method for the design of an emptying opening of the abovementioned type which makes it possible to ensure a good seal between the outer cover strip and the inner protective strip within the whole area of the emptying opening.
It is a further object to provide a method which in the aforementioned emptying opening makes it possible to seal the cover strip and the protective strip to each other right to the edge line of the emptying opening. This is particularly important in the part of the emptying opening where the breaking through of the protective layer commences when the cover strip is removed, since otherwise it can be difficult to break through and completely remove the part of the protective strip corresponding to the shape of the emptying opening.
These and other objects have been achieved in accordance with the invention in that a method for the sealing on a packing material provided with an emptying opening, of two thermoplastic layers located on opposite sides of the opening to each other by means of heat and pressure has been given the characteristic that in the emptying opening the one layer, by means of a pressure fluid, is made to rest against the other layer at the same time as the layers are heated to sealing temperature.
By means of the method in accordance with the invention it becomes possible to bring about a uniform and extensive contact and sealing between layers within the whole area of the emptying opening, at the same time as the limited, stronger heating along a part of the edge zone of the emptying opening ensures a distinct adhesion between the strips immediately adjoining the cut edge of the laminate so that an easy and complete tearing off of the area of the protective layer corresponding to the emptying opening is ensured when the cover strip is removed.
It is a further object of the present invention to provide an arrangement for the realization of the method. Such an arrangement in accordance with the invention comprises a heating device, the working surface of which can be pressed against a holding-up or support tool provided with a plane working surface. The heating device being heatable within a limited heating zone and being provided with an outlet for pressure medium.
The arrangement in accordance with the invention differs from similar known arrangements first and foremost in that it operates with a pressure fluid and in that it has a heating zone adapted to the shape of the emptying opening with a limited intensive heating region. As a result, a uniform pressure distribution is made possible which on heating and pressing of the protective layer against the cover layer ensures a complete shaping of the same over the whole surface of the emptying opening, at the same time as the limited intensive heating makes it possible to obtain the desired strong flattening out and resting against one edge of the emptying opening.
A preferred embodiment of the method and arrangement in accordance with the invention will now be described in more detail with special reference to the attached drawing which illustrates schematically the method and the arrangement in accordance with the invention.